There are some small regions in the United States which experience unexpected
high tornado frequency, and also areas with unexpected low frequency of
tornadoes.
For example, Hall County has a tornado density of 124.5 twisters per 1,000
square miles. However, Lancaster and Dodge counties have only 35.8 and
35.5 tornadoes per 1,000 square miles.

These "holes" in the broader map of tornado distribution and
concentration are mostly due to chance and randomness, not some physical
or topographical condition.